Gel electrophoresis is a common procedure for the separation of biological molecules, such as deoxyribonucleic acid (DNA), ribonucleic acid (RNA), polypeptides and proteins. In gel electrophoresis, the molecules are separated into bands according to the rate at which an imposed electric field causes them to migrate through a filtering gel.
The basic apparatus used in this technique consists of a gel enclosed in a glass tube or sandwiched as a slab between glass or plastic plates. The gel has an open molecular network structure, defining pores which are saturated with an electrically conductive buffered solution of a salt. These pores through the gel are large enough to admit passage of the migrating macromolecules.
The gel is placed in a chamber in contact with buffer solutions which make electrical contact between the gel and the cathode or anode of an electrical power supply. A sample containing the macromolecules and a tracking dye is placed on top of the gel. An electric potential is applied to the gel causing the sample macromolecules and tracking dye to migrate toward the bottom of the gel. The electrophoresis is halted just before the tracking dye reaches the end of the gel. The locations of the bands of separated macromolecules are then determined. By comparing the distance moved by particular bands in comparison to the tracking dye and macromolecules of known mobility, the mobility of other macromolecules can be determined. The size of the macromolecule can then be calculated.
The rate of migration of macromolecules through the gel depends upon three principle factors: the porosity of the gel; the size and shape of the macromolecule; and the charge density of the macromolecule. It is critical to an effective electrophoresis system that these three factors be precisely controlled and reproducible from gel to gel and from sample to sample. However, maintaining uniformity between gels is difficult because each of these factors is sensitive to many variables in the chemistry of the gel system.
Polyacrylamide gels are commonly used for electrophoresis. Polyacrylamide gel electrophoresis or PAGE is popular because the gels are optically transparent, electrically neutral and can be made with a range of pore sizes. The porosity of a polyacrylamide gel is in part defined by the total percentage of acrylamide monomer plus crosslinker monomer ("%T") it contains. The greater the concentration, the less space there is between strands of the polyacrylamide matrix and hence the smaller the pores through the gel. An 8% polyacrylamide gel has larger pores than a 12% polyacrylamide gel. An 8% polyacrylamide gel consequently permits faster migration of macromolecules with a given shape, size and charge density. When smaller macromolecules are to be separated, it is generally preferable to use a gel with a smaller pore size such as a 20% gel. Conversely for separation of larger macromolecules, a gel with a larger pore size is often used, such as an 8% gel.
Pore size is also dependent upon the amount of crosslinker used to polymerize the gel. At any given total monomer concentration, the minimum pore size for a polyacrylamide gel is obtained when the ratio of total monomer to crosslinker is about 20:1, (the usual expression for this ratio would be "5% C").
Several factors may cause undesirable variation in the pore size of gels. Pore size can be increased by incomplete gel polymerization during manufacture. Hydrolysis of the polyacrylamide after polymerization can create fixed negative charges and break down the crosslinks in the gel, which will degrade the separation and increase the pore size. An ideal gel system should have a reproducible pore size and no fixed charge (or at least a constant amount) and should be resistant to change in chemical characteristics or the pore size due to hydrolysis.
The size of the macromolecule varies between different macromolecules; the smaller and more compact the macromolecule the easier it will be for the macromolecule to move through the pores of a given gel. Given a constant charge density, the rate of migration of a macromolecule is inversely proportional to the logarithm of its size.
For accurate and reproducible electrophoresis, a given type of macromolecule should preferably take on a single form in the gel. One difficulty with maintaining uniformity of the shape of proteins during gel electrophoresis is that disulfide bonds can be formed by oxidation of pairs of cysteine amino acids. Different oxidized forms of the protein then have different shapes and, therefore, migrate through the gel run with slightly different mobilities (usually faster than a completely reduced protein, since the maximum stokes radius and minimum mobility should occur with a completely unfolded form). A heterogeneous mixture of forms leads to apparent band broadening. In order to prevent the formation of disulfide bonds, a reducing agent such as dithiothreitol (DTT) is usually added to the samples to be run. The shape of DNA and RNA macromolecules is dependent on temperature. In order to permit electrophoresis on temperature-dependent DNA and RNA molecules in their desired form, separations are done at a controlled temperature.
The charge density of the migrating molecule is the third factor affecting its rate of migration through the gel--the higher the charge density, the more force will be imposed by the electric field upon the macromolecule and the faster the migration rate subject to the limits of size and shape. In SDS PAGE electrophoresis, the charge density of the macromolecules is controlled by adding sodium dodecyl sulfate (SDS) to the system. SDS molecules associate with the macromolecules and impart a uniform charge density to them, substantially negating the effects of any innate molecular charge. Unlike proteins, the native charge density of DNA and RNA is generally constant, due to the uniform occurrence of phosphate groups. Thus, charge density is not a significant problem in electrophoresis of DNA and RNA.
SDS PAGE gels are usually poured and run at basic pH. The most common PAGE buffer system employed for the separation of proteins is that developed by Ornstein (1) and modified for use with SDS by Laemmli (2). Laemmli, U.K. (1970) Nature 227, 680-686. The Laemmli buffer system consists of 0.375 M tris (hydroxy methyl) amino-methane (Tris), titrated to pH 8.8. with HCl, in the separating gel. The stacking gel consists of 0.125 M Tris, titrated to pH 6.8. The anode and cathode running buffers contain 0.024 M Tris, 0.192 M glycine, 0.1% SDS. An alternative buffer system is disclosed by Schaegger and von Jagow. Schaegger, H. and von Jagow, G., Anal. Biochem. 1987, 166, 368-379. The stacking gel contains 0.75 M Tris, titrated to pH 8.45 with HCl. The separating gel contains 0.9 M Tris, titrated to pH 8.45 with HCl. The cathode buffer contains 0.1 M Tris, 0.1 M N-tris(hydroxymethyl)methylglycine (tricine), 0.1% SDS. The anode buffer contains 0.2 M Tris, titrated to pH 8.9 with HCl. For both of these systems Tris is the "common ion" which is present in the gel and in the anode and cathode buffers.
In the Laemmli system, the pH of the trailing phase in the stacking gel is about 8.9. In the separating gel, the trailing phase pH is about 9.7. At this pH, primary amino groups of proteins react readily with unpolymerized acrylamide, thiol groups are more subject to oxidation to disulfides, or reaction with unpolymerized polyacrylamide, than at neutral pH and acrylamide itself is subject to hydrolysis.
The shape of the DNA and RNA macromolecules is also dependent on a fourth important factor, temperature. The temperature-dependent shape of DNA and RNA is caused by the interaction of two macromolecules containing complementary sequences and the interaction of complementary sequences in a single macromolecule. Some techniques require that the DNA remain in its double-stranded form. Typically, such separations are done in Tris borate ethylene diamine tetra-acetic acid (TBE) buffer, consisting of 0.09 M Tris, 0.09 M boric acid, and 0.002 M ethylene diamine tetra-acetic acid (EDTA) on either polyacrylamide or agarose gels. In general, these separations are done at lower temperatures to maintain the double-stranded structure. In the absence of denaturants, DNA's and RNA's structure is fairly stable and not significantly affected by temperature.
In other techniques, dissociation of the two DNA strands (known as "melting") is utilized to effect the separation. Such methods require careful temperature control in order to produce a consistent separation. One method, non-isotopic single-strand conformational polymorphism ("Cold SSCP"), utilizes a dissociative sample buffer with heat to melt the strands, a TBE buffer, and a polyacrylamide gel. In Cold SSCP, temperatures of 4 to 35.degree. C. are used to allow variable-conformation renaturation to occur between mutant strands, and temperature changes of only a few degrees can significantly alter the number of mutants seen. See Hongyo, et al., Nucleic Acids Research, 21, 3637 (1993). Another method, employed in DNA sequence analysis, typically utilizes TBE buffers containing 6 to 8 M urea and/or 2 to 12 M formamide, and elevated temperatures. It is important that the temperature remain high enough--typically 45 to 55.degree. C.--to maintain fully melted DNA or RNA. Gels are usually polyacrylamide and sometimes substituted acrylamide polymers. For example, certain alkyl-substituted polyacrylamide gels are described in Shoor et al., U.S. Pat. No. 5,055,517.
These DNA and RNA separation methods are characterized by the use of continuous buffer systems, which use the same buffer species and generally, but not necessarily, in the same concentrations in the gel, the anode chamber, and the cathode chamber. These buffers usually are comprised of Tris and boric acid with EDTA added to inhibit hydrolytic enzyme activity. The TBE buffer system typically does not provide good stability when used in pre-cast gels, made and stored for periods of weeks at 4.degree. C. The polymer tends to break down, generating a fixed charge which leads to distortion particularly at the cathode end of the gel where resolution is especially important. Urea also tends to break down under alkaline pH at 4.degree. C. When large concentrations of urea are present, the ionic breakdown products can be present at a large enough concentration to disrupt the separation and cause loss of resolution.
Other buffer systems for DNA and RNA separations employ Tris/acetate, Tris/phosphate, and Tris/glycylglycine. While these buffer systems may be formulated near pH 7, the pK.sub.a of Tris causes them to shift to an alkaline pH during electrophoresis especially near the cathode. The applicants have found that the polyacrylamide and urea tend to break down during electrophoresis for DNA sequencing due to the high temperatures (50.degree. C.) employed for several hour runs when Tris is used as the buffering base. This breakdown leads to higher current and lower resolution than might be obtained with a neutral pH buffer system, so that the DNA sequence read length is reduced and read errors are increased.
The need for uniformity and predictability is magnified in precast electrophoresis gels which are manufactured by an outside vendor and then shipped to the laboratory where the electrophoresis will be performed. Precast gels must control the properties discussed above and they must be able to maintain this control throughout shipping and storage. The shelf life of many precast gels is limited by the potential for hydrolysis of acrylamide and/or buffer constitution during storage at the high pH of the gel buffer.
It is a disadvantage of a high pH gel that the polyacrylamide gel is subject to degradation by hydrolysis and has a limited shelf-life.
It is a further disadvantage of a high pH gel that proteins react readily with unpolymerized acrylamide which may interfere with subsequent analysis of the protein such as peptide sequencing.
It is a still further disadvantage of a high pH gel that thiol groups are subject to oxidation to disulfides causing a decreased resolution of separated macromolecules.
It is a further disadvantage of a high pH gel that buffer constituents such as urea break down readily.